Cooperative and submolecular dissipation mechanisms of sliding friction in complex organic systems

Abstract

Energy dissipation in single asperity sliding friction was directly linked to submolecular modes of mobility by intrinsic friction analysis, involving time-temperature superposition along with thermodynamic stress and reaction rate models. Thereby, polystyrene served as a representative tribological sample for organic and amorphous complex systems. This study reveals the significance of surface and subsurface (alpha-, beta-, and gamma-) relaxational modes, which couple under appropriate external conditions (load, temperature, and rate) with shear induced disturbances, and thus gives rise to material specific frictional dissipation. At low pressures and temperatures below the glass transition point, the phenyl pendant side groups of polystyrene, known for their preferential orientation at the free surface, were noticed to be the primary channel for dissipation of kinetic sliding-energy. While this process was found to be truly enthalpic (activation energy of 8 kcalmol), energy dissipation was shown to possess both enthalpic and cooperative entropic contributions above the loading capacity of the surface phenyl groups (9.9 kcalmol) or above the glass transition. Apparent Arrhenius activation energies of frictional dissipation of 22 and 90 kcalmol, respectively, and cooperative contributions up to 80% were found. As such, this study highlights issues critical to organic lubricant design, i.e., the intrinsic enthalpic activation barriers of mobile linker groups, the evaluation of cooperative mobility phenomena, and critical tribological parameters to access or avoid coupling between shear disturbances and molecular actuators.

Nanoscale Frictional Dissipation into Shear-Stressed Polymer Relaxations

“…For the above calculated values for contact radii at a sliding speed of 500 nm=s, we find that the interaction time varies from 34 to 44 ms. With activation energies for of E a ¼ 11 eV and of E a ¼ 0:4-0:7 eV (typical for various PAEKs [13][14][15][16]), we obtain temperature shifts of 1 order of magnitude below the observed effect. Even if we consider the (aforementioned) length scale of cooperativity for the time scale estimation [22], we do not obtain the measured shift.…”

Insight into reverse selectivity and relaxation behavior of poly[1-(trimethylsilyl)-1-propyne] by flux-lateral force and intrinsic friction microscopy

“…A solely activated process in a polymer such as PTMSP involves an isolated motional process, which is consistent with side chain rotation, or highly isolated backbone motions [24,27]. As we consider PTMSP in a temperature range far below its decomposition temperature and glass transition of 330 • C [41] we can neglect translational modes of relaxation, which are also known to be highly cooperative [24,25,27].…”

“…Although there is a small decreasing trend noticeable with decreasing film thickness, the error bar is relatively large, but a modestly decreasing trend is apparent. It is important to note that the energy analysis required only horizontal shifting, as discussed in detail elsewhere [24,27], indicating that the underlying thermal relaxation process is entirely enthalpic, i.e., can be attributed to an activated molecular or submolecular process, without significant entropic cooperative contributions. A solely activated process in a polymer such as PTMSP involves an isolated motional process, which is consistent with side chain rotation, or highly isolated backbone motions [24,27].…”

“…It is important to note that the energy analysis required only horizontal shifting, as discussed in detail elsewhere [24,27], indicating that the underlying thermal relaxation process is entirely enthalpic, i.e., can be attributed to an activated molecular or submolecular process, without significant entropic cooperative contributions. A solely activated process in a polymer such as PTMSP involves an isolated motional process, which is consistent with side chain rotation, or highly isolated backbone motions [24,27]. As we consider PTMSP in a temperature range far below its decomposition temperature and glass transition of 330 • C [41] we can neglect translational modes of relaxation, which are also known to be highly cooperative [24,25,27].…”

“…From previous macroscopic PTMSP studies [8], insignificant changes in the storage modulus (E ), the loss modulus (E ), and the loss tangent (tan ı), over a temperature range of −150 • C to 200 • C provide no effective means to determine relaxation properties, such as activation energies. With local and highly sensitive scanning, the IFA technique, which has provided activation energies of molecular relaxations for a variety of polymers and molecular glasses [25][26][27][28], can be used to determine local and faint relaxation phenomena, and thereby, molecular relaxation modes can be identified. Employing IFA complementary to theoretical simulations and F-LFM, we will demonstrate that the membrane permeation is strongly affected by fundamental changes in the polymer matrix induced by the permeant sorption and aging and is not primarily related to the original molecular mobility and structure of the virgin membrane.…”